Some people get exposed to the ugly side of JW's more than others when it comes to shunning or marking or cliques or double standards with favoritism for the "in" crowd and harsh treatment for the "out" crowd.
In other cases, a person is kicked out for breaking various rules and that leads to them investigating the religion. Or similarly, a person who is in may be disturbed by a particular change in the teachings because the way it was understood was ingrained in him or very important to him. Many others don't pay attention to the details and just figure that smarter/more spiritual people than them have it all figured out.